Polyimide/aliphatic polyester copolymers

ABSTRACT

A new copolymer composition comprising the hard segment of a polyimide and the soft segment of an oligomeric aliphatic polyester. The new polyimide copolymer membranes have exhibited high thermal stability for separation of an aromatic/saturate mixture. These new membranes have higher aromatic/saturate selectivity than polyurethanes.

BACKGROUND

The present invention relates to a new composition of matter for theseparation of aromatics from saturates.

The use of membranes to separate aromatics from saturates has long beenpursued by the scientific and industrial community and is the subject ofnumerous patents.

U.S. Pat. No. 3,370,102 describes a general process for separating afeed into a permeate stream and a retentate stream and utilizes a sweepliquid to remove the permeate from the face of the membrane to therebymaintain the concentration gradient driving force. The process can beused to separate a wide variety of mixtures including various petroleumfractions, naphthas, oils, hydrocarbon mixtures. Expressly recited isthe separation of aromatics from kerosene.

U.S. Pat. No. 2,958,656 teaches the separation of hydrocarbons by type,i.e., aromatic, unsaturated, saturated, by permeating a portion of themixture through a non-porous cellulose ether membrane and removingpermeate from the permeate side of the membrane using a sweep gas orliquid. Feeds include hydrocarbon mixtures, e.g., naphtha (includingvirgin naphtha, naphtha from thermal or catalytic cracking, etc.).

U.S. Pat. No. 2,930,754 teaches a method for separating hydrocarbons,e.g., aromatic and/or olefins from gasoline boiling range mixtures, bythe selective permeation of the aromatic through certain non-porouscellulose ester membranes. The permeated hydrocarbons are continuouslyremoved from the permeate zone using a sweep gas or liquid.

U.S. Pat. No. 4,115,465 teaches the use of polyurethane membranes toselectively separate aromatics from saturates via pervaporation.

Compared to distillation, membrane permeation can lead to considerableenergy savings. A membrane can separate a mixture of aromatics andsaturates, e.g., a heavy cat naphtha, into a high-octane, mainlyaromatic permeate and a high-octane, mainly saturated retentate. Bothpermeate and retentate are more valuable than the starting heavy catnaphtha.

SUMMARY OF THE INVENTION

The present invention is a new composition of matter and its use in aprocess for separating aromatics from feeds which are mixtures ofaromatics and non-aromatics. The composition of matter includes a hardsegment having a glass transition temperature (Tg) of greater than 130°C. and a soft segment having a Tg less than the Tg of the hard segment.The composition is formed into a membrane which includes a copolymercomposition with the hard segment of a polyimide and the soft segment ofan oligomeric aliphatic polyester, wherein the hard and soft segmentsare alternating, the polyimide is derived from a dianhydride and adiamine, and the oligomeric aliphatic polyester is a polyadipate, apolysuccinate, a polymalonate, a polyoxalate or a polyglutarate.

In a preferred embodiment, the dianhydride has between 8 and 20 carbons,the diamine has between 2 and 30 carbons, and the oligomeric aliphaticpolyester is a polyadipate or a polysuccinate.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 shows the synthesis and composition of the copolymer containingthe hard segment of the polyurea/urethane with 2 chlorines and the softsegment of polyethylene adipate with a molecular weight of 2000 (PEA2000).

FIG. 2 shows that the soft segment of the polyurethane controlssorption, i.e., selectivity and permeability.

FIG. 3 shows three polyurethanes with different hard segments.

FIG. 4 shows that chlorinated polyurethanes have higher thermalstability than the polyurethane without chlorines, the polyurethanecontaining the hard segment with 6 chlorines has the highest thermalstability among the three polyurethanes investigated, and change in hardsegment does not affect selectivity and permeability.

FIG. 5 shows the synthesis and composition of a new copolymer of thepresent invention containing the hard segment of polyimide derived frompyromellitic dianhydride (PMDA) and methylene di-o-chloroaniline (MOCA)and the soft segment of polyethylene adipate with a molecular weight of2000 (PEA 2000).

FIG. 6 shows the toluene/isooctane selectivity and permeability (flux)for a polyimide copolymer membrane of the present invention containingthe soft segment of PEA 2000, and higher selectivity and thermalstability with this membrane than with the polyurethane membranecontaining the hard segment with 6 chlorines.

FIG. 7 shows the toluene/isooctane selectivity and permeability foranother polyimide copolymer membrane of the present invention containingthe soft segment of polyethylene adipate with a molecular weight of 1000(PEA 1000).

FIG. 8 shows that a polyimide copolymer with single polyadipate hashigher selectivity but lower permeability than that with mixedpolyadipates.

FIG. 9 shows that increasing polyimide hard segment fraction improvesoverall aromatics/saturates selectivity but decreases permeability(flux).

FIG. 10 shows that increasing polyimide hard segment fraction improvestoluene/n-octane selectivity but reduces permeability (flux).

FIG. 11 shows that the new membranes of the present invention,polyimide/polysuccinate and polyimide/polyadipate copolymer membranes,give improved selectivity and flux vs. the membranes investigated.

FIG. 12 shows the toluene/isooctane and toluene/n-octane selectivitiesand permeability for a polyimide copolymer of the present invention,containing the soft segment of polyethylene adipate with a molecularweight of 2000 (PEA 2000), cured at 260° C. for 5 minutes.

FIG. 13 shows the toluene/isooctane and toluene/n-octane selectivitiesand permeability for a polyimide copolymer of the present invention,containing the soft segment of polyethylene adipate with a molecularweight of 3000 (PEA 3000), cured at 260° C. for 12 minutes.

FIG. 14 shows that a thinner polyimide copolymer membrane has maintainedthe selectivity and permeability of a thicker one.

FIG. 15 shows that a thinner polyimide copolymer membrane has increasedflux and maintained selectivity vs. a thicker one.

FIG. 16 shows that decreasing soft segment molecular weight increasesselectivity but decreases permeability (flux).

FIG. 17 shows that selectivity correlates with permeability forpolyimide copolymers with various polyadipate molecular weights.

FIG. 18 shows that increasing curing severity improves toluene/isooctaneselectivity but reduces permeability (flux).

FIG. 19 shows that increasing curing severity improves toluene/n-octaneselectivity but reduces permeability (flux).

FIG. 20 shows that selectivity correlates with permeability forpolyimide copolymers cured at different conditions.

FIG. 21 shows the toluene/isooctane selectivities and permeabilities(fluxes) for the polyimide copolymer membranes, containing the softsegment of PEA 2000, cured at 260° C. for 4 and 5 minutes.

FIG. 22 shows the toluene/n-octane selectivities and permeabilities(fluxes) for the polyimide copolymer membranes, containing the softsegment of PEA 2000, cured at 260° C. for 4 and 5 minutes.

FIG. 23 shows that a polyimide copolymer of the present invention has ahigher thermal decomposition temperature than a polyurethane membrane.

FIG. 24 shows the thermal decomposition temperature for PEA 2000 diol inair.

FIG. 25 shows the thermal decomposition temperature for PEA 4000 diol inair.

FIG. 26 shows the thermal decomposition temperature for PEA 2000 diol innitrogen.

FIG. 27 shows the thermal decomposition temperature for PEA 4000 diol innitrogen.

FIG. 28 shows that a polyimide copolymer of the present invention hasstable selectivity and permeability (flux) for at least 6 days at 210°C.

DESCRIPTION OF THE PREFERRED EMBODIMENT

The present invention is new polyimide copolymers for membranes toseparate aromatics from feed streams of aromatics and non-aromatics. Thecopolymer contains alternating hard and soft segments.

We have synthesized a copolymer containing the hard segment of thepolyurea/urethane with 2 chlorines and the soft segment of thepolyethylene adipate with a molecular weight of 2000 (PEA 2000). Thehard and soft segments ar alternating in the copolymer. FIG. 1 shows thesynthesis and composition of the copolymer. In the synthesis, one moleof polyethylene adipate diol reacts with 2 moles of methylenediphenylisocyanate (MDI). That is, polyethylene adipate abbreviated asPEA is end-capped with MDI to make a pre-polymer. This pre-polymer isthen chain-extended with di-o-chloroaniline (MOCA) to make the copolymercontaining the soft segment of PEA and the hard segment ofpolyurea/urethane hard segment with 2 chlorines (MDI-MOCA-MDI).

We have elucidated the function of soft and hard segments for membraneseparation. The soft segment governs sorption of hydrocarbons, i.e.selectivity and permeability whereas the hard segment provides thethermal stability of the membrane. We have studied the glass transitiontemperatures, Tg, of the soft and hard segments of the copolymer shownin FIG. 1. The diagram on the left hand side of FIG. 2 shows Tg resultsfor the soft segment. The Tg of the soft segment is reduced with thesorption of toluene in the polyurethane membrane. This means that thehydrocarbon is sorbed in the soft segment. The diagram on the right handside shows Tg results for the hard segment. The Tg of the hard segmentis unchanged with the sorption of mesitylene in the membrane. This meansthat the hydrocarbon does not penetrate the hard segment. Mesityleneinstead of toluene was used in the Tg experiments since the Tg of thehard segment is higher than the boiling point of toluene. As shown inthese diagrams, the hard segment has a much higher Tg than the softsegment. These Tg results show that the soft segment governs sorption.That is, the soft segment governs selectivity and permeability.

Table 1 shows that the hard segment provides the thermal stability ofthe membrane. This table gives two evidences to support this conclusion.The first evidence is that a chlorinated hard segment gives a bettermembrane stability than a hard segment without chlorine. The chlorinatedhard segment from MDI and MOCA gives a membrane stability of 150° C.However, the hard segment without chlorine, which is from MDI andmethylene dianiline, results in a membrane stability of only 100° C.Both membranes have the same soft segment of polyethylene adipate with amolecular weight of 2000. The chlorinated hard segment has a higherglass transition temperature than the hard segment without chlorine. Theenhanced membrane stability and higher glass transition temperature forthe chlorinated hard segment are presumably due to the dipole-dipoleinteraction of the chlorine groups. The second evidence is thatcrosslinking of hard segment improves the thermal stability of themembrane. As shown in the lower part of this table, the hard segment isfrom toluene diisocyanate (TDI) and phenylene diamine. Crosslinking ofthe hard segment is via the use of 1,1,1-trimethylol-ethane to replace25 mole % of phenylene diamine in the hard segment. The crosslinked hardsegment gives a better membrane stability than the hard segment withoutcrosslinking. Both membranes have the same soft segment ofpolydiethyleneglycol adipate with a molecular weight of 2000.

                  TABLE 1                                                         ______________________________________                                        HARD SEGMENT OF POLYURETHANE INFLUENCES                                       THERMAL STABILITY                                                                            Membrane Glass Transition                                                     Stability (°C.)                                                                 Temperature (°C.)                              ______________________________________                                        Chlorinated Hard Segment                                                      Gives Better Stability                                                        --Chlorinated Hard Segment                                                                     150        125                                               (MDI/MOCA)                                                                    --Hard Segment Without                                                                         100         95                                               Chlorine (MDI/Methylene                                                       Dianiline)                                                                    Crosslinking of Hard Segment                                                  Improves Stability                                                            --Crosslinked Hard Segment                                                                     120        125                                               (TDI/Phenylene Diamine)                                                       --Hard Segment Without                                                                          80        --                                                Crosslinking                                                                  ______________________________________                                    

We have also synthesized a copolymer containing the hard segment of thepolyurea/urethane with 6 chlorines and the soft segment of polyethyleneadipate with a molecular weight of 2000 (PEA 2000) via the synthesismethod described above, but using methylene dichlorophenylisocyanate(dichloro MDI) instead of methylene diphenylisocyanate (MDI). FIG. 3shows the three polyurethanes with the same soft segment but with threedifferent hard segments. The soft segment was the polyethylene adipate(PEA) with a molecular weight of 2000. The first polyurethane had a hardsegment without chlorine, which was MDI/methylene dianiline (MDA)/MDI.The second polyurethane had a hard segment with 2 chlorines, which wasMDI/MOCA/MDI. The third polyurethane had a hard segment with 6chlorines, which was dichloro MDI/MOCA/dichloro MDI. This is theadvanced polyurethane.

We have evaluated the three polyurethane membranes to separate a mixturecontaining toluene and isooctane in a pervaporation apparatus. Theinitial mixture contains about equal weights of the two hydrocarbons.The pervaporation apparatus is a cell separated into two compartments bya porous metal plate, on which the membrane is supported. During apervaporation experiment the toluene-isooctane mixture is circulatedthrough the upper compartment at the desired temperature. The lowercompartment is kept at reduced pressure. The permeate is collected in atrap cooled with dry ice-acetone or isopropanol and periodicallyanalyzed by gas chromatography.

FIG. 4 compares the performance for these three polyurethane membraneswith different hard segments in the pervaporation of thetoluene/isooctane feed. In the lower part of this figure, we plotnormalized flux, i.e., permeability in the unit of kilogram of permeateper meter square membrane area per day for a normalized membranethickness of 1 micron (Kg·μM/M² /D), as a function of temperature. Thehard segment without chlorine gave a membrane stability of 100° C. Thehard segment with 2 chlorines resulted in a membrane stability of 150°C. However, the advanced polyurethane containing the hard segment with 6chlorines had a thermal stability of about 170° C. These results havereinforced our finding that the hard segment provides thermal stability.The advanced polyurethane with the highly chlorinated hard segment hadthe highest thermal stability among the polyurethanes investigated. Thisfigure shows that the permeability was identical for these threemembranes at a given temperature, at which they were stable. This wasdue to the fact that they had the same soft segment. The upper part ofthis figure shows that the selectivity was about the same for thesethree membranes at a given temperature, at which they were stable. Thiswas also due to the fact that they had the same soft segment. Therefore,change in hard segment did not affect selectivity and permeabilitysignificantly. These results have reinforced our findings that the softsegment governs selectivity and permeability, and the hard segmentprovides thermal stability.

Through our finding that the hard segment provides thermal stability,which is enhanced with increasing hard segment Tg, we have synthesizednew copolymers of the present invention containing polyimide with highTg as the hard segment to improve thermal stability. FIG. 5 shows thesynthesis and composition of an example of the new copolymers containingthe hard segment of polyimide derived from pyromellitic dianhydride(PMDA) and methylene di-o-chloroaniline (MOCA) and the soft segment ofpolyethylene adipate with a molecular weight of 2000 (PEA 2000). In thesynthesis, one mole of polyethylene adipate (PEA) is reacted with twomoles of pyromellitic dianhydride (PMDA) to make pre-polymer in theendcapping step. Then, one mole of the pre-polymer reacts with one moleof methylene di-o-chloroaniline (MOCA) to make a copolymer containingPEA soft segment and polyamic acid hard segment in the chain-extensionstep. Finally, heating of the copolymer at 260°-300° C. for about 0.5hour leads to the new copolymer containing PEA soft segment andpolyimide hard segment. The heating step converts the polyamic acid hardsegment to the polyimide hard segment via the imide ring closure withremoval of water.

In the synthesis, dimethyl formamide (DMF) is used as solvent in thechain-extension step. DMF is a preferred solvent, but other suitablesolvents may be used. This gives a concentrated solution of the polyamicacid/polyadipate copolymer in DMF. The new polyimide copolymer membranecan be prepared by casting the solution on a glass plate or a poroussupport, adjusting the thickness by means of a casting knife, and dryingthe membrane first at room temperature to remove most of the solvent,then at 120° C. overnight. The membrane is then removed from the glassplate via soaking in water. Finally, heating of the membrane at 300° C.for about 0.5 hour results in the polyimide copolymer membrane.

The new polyimide copolymer membranes can be used for the separation ofaromatics from saturates. In separation experiments similar to thosedescribed above, the membranes are employed to separate a feed mixturecontaining 50 wt % toluene and 50 wt % isooctane or a mixture containing10 wt % toluene, 40 wt % p-xylene, 20 wt % isooctane, and 30 wt %n-octane in the pervaporation apparatus.

The membranes are useful for the separation of aromatics from saturatesin petroleum and chemical streams, and have been found to beparticularly useful for the separation of large substituted aromaticsfrom saturates as are encountered in heavy cat naphtha streams. Otherstreams which are also suitable feed streams for aromatics fromsaturates separation are intermediate cat naphtha streams boiling at93°-60° C., light aromatics content streams boiling in the C₅ -150° C.range, light catalytic cycle oil boiling in the 200°-345° C. range aswell as streams in chemical plants which contain recoverable quantitiesof benzene, toluene, xylenes (BTX) or other aromatics in combinationwith saturates. The separation techniques which may successfully employthe membranes of the present invention include perstraction andpervaporation.

Perstraction involves the selective dissolution of particular componentscontained in a mixture into the membrane, the diffusion of thosecomponents through the membrane and the removal of the diffusedcomponents from the downstream side of the membrane by the use of aliquid sweep stream. In the perstractive separation of aromatics fromsaturates in petroleum or chemical streams (particularly heavy catnaphtha streams) the aromatic molecules present in the feedstreamdissolve into the membrane film due to similarities between the membranesolubility parameter and those of the aromatic species in the feed. Thearomatics then permeate (diffuse) through the membrane and are sweptaway by a sweep liquid which is low in aromatic content. This keeps theconcentration of aromatics at the permeate side of the membrane film lowand maintains the concentration gradient which is responsible for thepermeation of the aromatics through the membrane.

The sweep liquid is low in aromatics content so as not to itselfdecrease the concentration gradient. The sweep liquid is preferably asaturated hydrocarbon liquid with a boiling point much lower or muchhigher than that of the permeated aromatics. This is to facilitateseparation, as by simple distillation. Suitable sweep liquids,therefore, would include, for example, C₃ to C₆ saturated hydrocarbonsand lube basestocks (C₁₅ -C₂₀).

The perstraction process is run at any convenient temperature,preferably as low as possible.

The choice of pressure is not critical since the perstraction process isnot dependent on pressure, but on the ability of the aromatic componentsin the feed to dissolve into and migrate through the membrane under aconcentration driving force. Consequently, any convenient pressure maybe employed, the lower the better to avoid undesirable compaction, ifthe membrane is supported on a porous backing, or rupture of themembrane, if it is not.

If C₃ or C₄ sweep liquids are used at 25° C. or above in liquid state,the pressure must be increased to keep them in the liquid phase.

Pervaporation, by comparison, is run at generally higher temperaturesthan perstraction and relies on vacuum on the permeate side to evaporatethe permeate from the surface of the membrane and maintain theconcentration gradient driving force which drives the separationprocess. As in perstraction, the aromatic molecules present in the feeddissolve into the membrane film, migrate through said film and merge onthe permeate side under the influence of a concentration gradient.Pervaporative separation of aromatics from saturates can be performed ata temperature of about 25° C. for the separation of benzene from hexanebut for separation of heavier aromatic/saturate mixtures, such as heavycat naphtha, higher temperatures of at least 80° C. and higher,preferably at least 100° C. and higher, more preferably 120° C. andhigher should be used. Temperatures of about 210° C. have beensuccessfully used with membranes of the present invention, the maximumupper limit being that temperature at which the membrane is physicallydamaged. Vacuum on the order of 1-50 mm Hg is pulled on the permeateside. The vacuum stream containing the permeate is cooled to condenseout the highly aromatic permeate. Condensation temperature should bebelow the dew point of the permeate at a given vacuum level.

The membrane itself may be in any convenient form utilizing anyconvenient module design. Thus, sheets of membrane material may be usedin spiral wound or plate and frame permeation cell modules. Tubes andhollow fibers of membranes may be used in bundled configurations witheither the feed or the sweep liquid (or vacuum) in the internal space ofthe tube or fiber, the other material obviously being on the other side.

When the membrane is used in a hollow fiber configuration with feedintroduced on the exterior side of the fiber, the sweep liquid flows onthe inside of the hollow fiber to sweep away the permeated highlyaromatic species, thereby maintaining the desired concentrationgradient. The sweep liquid, along with the aromatics contained therein,is passed to separation means, typically distillation means, however, ifa sweep liquid of low enough molecular weight is used, such as liquefiedpropane or butane, the sweep liquid can be permitted to simplyevaporate, the liquid aromatics being recovered and the gaseous propaneor butane (for example) being recovered and reliquefied by applicationof pressure or lowering of temperature.

It has been observed that the new polyimide copolymer membranes canseparate toluene from isooctane and toluene from n-octane, showing goodselectivity and permeability. These membranes have highertoluene/isooctane and toluene/n-octane selectivities than thepolyurethane membranes with the same polyadipate soft segment. Thesemembranes have exhibited the highest thermal stability, at least 210°C., among the membranes investigated for pervaporation separation of thefeed mixtures described above.

The copolymer composition of the present invention comprises the hardsegment of a polyimide and the soft segment of an oligomeric aliphaticpolyester. The polyimide is derived from a dianhydride and a diamine,and the oligomeric aliphatic polyester is a polyadipate, apolysuccinate, a polymalonate, a polyoxalate or a polyglutarate.

In a preferred embodiment, the dianhydride has between 8 and 20 carbonsand the diamine has between 2 and 30 carbons, and the oligomericaliphatic polyester is a polyadipate or a polysuccinate. The dianhydrideis preferred to be an aromatic compound. Non-limiting examples includepyromellitic dianhydride, 3,3',4,4'-benzophenone tetracarboxylicdianhydride, 4,4'-(hexafluoroisopropylidene)-bis(phthalic anhydride),4,4'-oxydiphthalic anhydride, diphenylsulfone-3,3',4,4'-tetracarboxylicdianhydride. Non limiting examples of diamine include phenylene diamine,methylene dianiline (MDA), methylene di-o-chloroaniline (MOCA),methylene bis (dichloroaniline) (tetrachloro MDA), methylenedicyclohexylamine (H₁₂ -MDA), methylene dichlorocyclohexylamine (H₁₂-MOCA), methylene bis (dichlorocyclohexylamine) (tetrachloro H₁₂ -MDA),4,4'-(hexafluoroisopropylidene)-bisaniline (6F diamine),3,3'-diaminophenyl sulfone (3,3'DAPSON), 4,4'-diaminophenyl sulfone(4,4'DAPSON), 4,4'-dimethyl-3,3'-diaminophenyl sulfone(4,4'-dimethyl-3,3' DAPSON), 2,4-diamino cumene, methylbis(di-o-toluidine), oxydianiline (ODA), bisaniline A, bisaniline M,bisaniline P, thiodianiline, 2,2-bis[4-(4-aminophenoxy) phenyl] propane(BAPP), bis[4-(4-aminophenoxy phenyl) sulfone (BAPS),4,4'-bis(4-aminophenoxy) biphenyl (BAPB), 1,4-bis(4-aminophenoxy)benzene (TPE-Q), and 1,3-bis(4-aminophenoxy) benzene (TPE-R).

U.S. Pat. Nos. 4,233,435 and 4,307,226 (equivalent to GB No. 2,075,998)to General Electric Company and Japanese Patent Nos. 56,014,528;56,062,823; 56,036,192; 56,036,561; 56,076,591 and 57,094,024 to NittoElectric Industrial Co., Ltd. are not related to membranes forseparation, and they are cited herewith for reference.

U.S. Pat. No. 4,233,435 discloses the production of a polyesterimideresin by reacting a mixture of a diamine, an anhydride, a dihydricalcohol containing 2 esterifiable hydroxyl groups (e.g., ethyleneglycol), a polyhydric alcohol containing ≧3 esterifiable hydroxyl groups(e.g., tris[2-hydroxyethyl] isocyanurate), a lower dialkyl ester ofterephthalic and/or isophthalic acid and a monohydric alcohol (e.g.,1-decanol). The resin is used as a coating, e.g., on electricalconductors such as wire.

As disclosed in U.S. Pat. No. 4,307,226 (equivalent to GB No.2,075,998), a polyesterimide is mixed with ethylene glycol and heated toincorporate the glycol in the polyesterimide by transesterification sothat on cooling a clear homogeneous resin is obtained, which is solublein a glycol monoether for wire coating applications. The polyesterimideis formed by reacting a mixture of a diamine, an anhydride, anesterifiable dihydric alcohol, a polyhydric alcohol containing ≧3esterifiable hydroxyl groups and a di(lower alkyl) terephthalate orisophthalate.

Japanese Patent No. 56,014,528 discloses the preparation of a polyesteror polyesterimide resin for electrically insulating coatings by reactinga mixture of terephthalic acid optionally blended with up to 50 mole %of a polycarboxylic acid, a polyhydric alcohol containing ≧2esterifiable hydroxyl groups and optionally a diamine in the presence ofat least one esterifying catalyst (e.g. dibutyltin dilaurate).

The reactants used in U.S. Pat. Nos. 4,233,435 and 4,307,226 andJapanese Patent No. 56,014,528 do not produce polymers containing thesoft segment of an oligomeric aliphatic polyester as disclosed in thepresent invention. Because the reactants have short chains, it appearsthat the resulting polymers have reasonably rigid chains with about thesame degree of chain flexibility and thus a single glass transitiontemperature (Tg) for each polymer and do not have hard and softsegments. In addition, the synthesis procedures disclosed in these threepatents should result in polyesterimides with random structures sinceboth the esters and alcohols can react with the dianhydridessimultaneously. As disclosed in the present invention, the copolymerssynthesized from an oligomeric aliphatic polyester, a dianhydride and adiamine via the sequential end-capping and chain-extension steps containthe hard segment of a polyimide and the soft segment of the oligomericaliphatic polyester, in which the hard and soft segments arealternating. These copolymers have two Tg values, one for the hardsegment domain and the other for the soft segment domain. With thefunction that the hard segment provides thermal stability and the softsegment governs selectivity and permeability, these copolymers offersuperior membrane materials for the separation of the feed mixturecontaining aromatics and saturates. Thus, the copolymers of the presentinvention are different from the polymers disclosed in these threepatents.

Japanese Patent No. 56,062,823 discloses the synthesis of apolyesteramide-imide or polyesterimide for use as electric insulatingmaterial, adhesive, printed circuit board, lamination material, paint,etc., by modifying a polyester with 1,2,3,4-butanetetracarboxylic acidand an aliphatic polyamine containing ≧2 amino groups. As disclosed inJapanese Patent No. 56,036,192, a polyesteramide-imide or polyesterimideis used for printed circuit board. Japanese Patent No. 56,036,561discloses the use of a polyesteramide-imide or polyesterimide for heatresistant adhesive. Japanese Patent No. 56,076,591 claims the use of apolyesteramide-imide or polyesterimide for the insulating material forcircuit board. As claimed in Japanese Patent No. 57,094,024, apolyesteramide-imide or polyesterimide is employed in the laminate boardmanufacture. As disclosed in these five Japanese patents, both1,2,3,4-butanetetracarboxylic acid and an aliphatic polyamine are usedin the modification of a polyester. Since both the acid and the esterfunctionality (ester linkage) of the polyester can react with thepolyamine simultaneously, the resulting polyesteramide-imide orpolyesterimide should have a random structure. Thus, the copolymers ofthe present invention containing the alternating polyimide hard segmentsand oligomeric aliphatic polyester soft segments, which are synthesizedfrom an oligomeric aliphatic polyester diol, a dianhydride and adiamine, are different from the polyesteramide-imides or polyesterimidesdisclosed in these five Japanese patents.

The invention is illustrated further by the following examples, which,however, are not to be taken as limiting in any respect. All parts andpercentages, unless expressly stated to be otherwise, are by weight.

EXAMPLE 1 Synthesis of the Polyimide Copolymer Containing thePolyethylene Adipate Soft Segment with a Molecular Weight of 2000

To 10 g (0.005 mole) of polyethylene adipate diol with a molecularweight of 2000 (PEA 2000) heated (about 80° C.) under N₂ in a reactorwas added 2.18 g (0.01 mole) of pyromellitic dianhydride (PMDA) withstirring. The temperature was increased to about 100° C., and thestirring continued for about 4 hours to complete the end-capping step.To the reactor content was added 5 g of N,N-dimethyl formamide (DMF),and the temperature was dropped to about 80° C. with stirring for about0.5 hour. To this reactor content was added 1.34 g (0.005 mole)methylene di-o-chloroaniline (MOCA) in 3 g DMF solution dropwise. Untila very viscous solution resulted, which indicated the chain-extensionreaction, additional DMF, about 38 g, was added, and the solution wasthen cooled to room temperature. The resulting solution containing thecopolymer with the polyamic acid hard segment and the polyethyleneadipate soft segment had suitable consistency for solution casting inthe preparation of membranes.

The resulting solution was centrifuged for about 5 minutes. Followingcentrifugation, a membrane was knife-cast onto a glass plate with aknife gap setting of 13 mils. DMF was allowed to evaporate from themembrane in a hood at ambient conditions over a period of about 17hours. The membrane was then dried in an oven at 120° C. overnight. Themembrane was then removed from the glass plate by soaking it in a waterbath. Finally, the membrane was cured, by heating it from roomtemperature to 300° C., maintaining it at this temperature for about 0.5hour and then cooling it to room temperature in the curing step, toobtain the polyimide copolymer membrane with the PEA soft segment. Theresulting membrane had a thickness of about 94 microns and apolyimide/polyadipate weight ratio of 25/75. The curing step convertedthe polyamic acid hard segment to the polyimide hard segment via theimide ring closure with removal of water. The polyimide copolymer wasinsoluble in DMF.

EXAMPLE 2 Pervaporation Results for the Polyimide Copolymer MembraneContaining the Polyethylene Adipate Soft Segment With a Molecular Weightof 2000

The resulting membrane described in Example 1 was evaluated foraromatic/saturate separation with the feed mixture of 50 wt % tolueneand 50 wt % isooctane in the pervaporation apparatus described above.FIG. 6 shows the toluene/isooctane selectivity and permeability for thepolyimide copolymer membrane in comparison with those for a polyurethanemembrane which contained the same soft segment of PEA 2000 and thehighly chlorinated hard segment with 6 chlorines, as shown in FIG. 3.The highly chlorinated polyurethane membrane had the same selectivityand permeability as the polyurethane membrane without Cl, as shown inFIG. 3, containing the same soft segment; however, the former had a highthermal stability of about 170° C. whereas the latter had a stability ofabout 100° C. As shown in the figure, the polyimide copolymer membranehad about the same permeability as the highly chlorinated polyurethanemembrane at a given temperature up to about 170° C. However, thepolyimide copolymer membrane had higher selectivity than the chlorinatedpolyurethane membrane. This polyimide copolymer membrane was stable atleast up to 210° C., which was the maximum temperature capability of thepervaporation apparatus. Thus, this membrane has exhibited the highestthermal stability among the membranes investigated. The high thermalstability of this membrane was presumably due to its polyimide hardsegment having a high glass transition temperature (Tg) of about 340° C.as determined from differential scanning calorimetry (DSC). This Tg wasmuch higher than that of the hard segment of the chlorinatedpolyurethane membrane, about 130° C. However, the soft segment Tg valuesfor the polyimide copolymer and polyurethane membranes were about thesame, about -25° C. Thus, these results have reinforced our finding thatthe hard segment provides thermal stability.

In addition to the thermal stability and selectivity advantagesdescribed above for the polyimide copolymer membrane vs. thepolyurethane membrane, the former membrane had better solvent resistancethan the latter. For example, the polyimide copolymer membrane wasinsoluble in DMF, whereas the polyurethane membrane was soluble in thesolvent.

EXAMPLE 3 Synthesis of the Polyimide Copolymer Containing thePolyethylene Adipate Soft Segment with a Molecular Weight of 1000

The procedure was the same as that described in Example 1 exceptpolyethylene adipate diol with a molecular weight of 1000 (PEA 1000) wasused instead of PEA 2000. The resulting membrane had a thickness ofabout 45 microns.

EXAMPLE 4 Pervaporation Results for the Polyimide Copolymer MembraneContaining the Polyethylene Adipate Soft Segment with a Molecular Weightof 1000

The resulting membrane described in Example 3 was evaluated in a similarway as described in Example 2. FIG. 7 shows the toluene/isooctaneselectivity and permeability for the polyimide copolymer membranecontaining the soft segment of PEA 1000. The selectivity decreased fromabout 40 at 100° C. to 14 at 150° C., and it remained fairly constantfrom 150° C. to 190° C. On the other hand, the permeability increasedsignificantly from about 200 Kg·μM/M² /D at 100° C. to 1800 Kg·μM/M² /Dat 150° C., and it increased slightly to about 2500 Kg·μM/M^(2/D) at190° C. The constant selectivity and the slight permeability increasefrom 150° C. to 190° C. were presumably due to the constant degree ofswelling or solubility for this membrane reached at this temperaturerange. At the constant swelling or solubility, the solubility ratiobetween toluene and isooctane should be fairly constant, and theirdiffusivity ratio was also fairly constant because the diffusivitieswere primarily a function of solubility at a given temperature. Theseresulted in the constant selectivity. The constant selectivity behaviorgives a reasonably good selectivity at high temperatures, and this isdesirable for high-temperature separations. The slight permeabilityincrease was due to the increase of the diffusivities with increasingtemperature in the absence of the solubility increase with temperature.

As shown in FIG. 7, the polyimide copolymer with the PEA 1000 softsegment had higher toluene/isooctane selectivity but lower permeabilitythan the polyurethane without Cl, as shown in FIG. 3, containing thesoft segment of PEA 2000. In comparison of this figure with FIG. 6, thispolyimide copolymer also had higher selectivity but lower permeabilitythan the polyimide copolymer with the soft segment of PEA 2000. Thehigher selectivity and lower permeability with the PEA 1000 soft segmentwere presumably due to narrower inter-segmental spacing, i.e., a tighterpolymer matrix vs. the case with PEA 2000 soft segment.

EXAMPLE 5 Synthesis of the Polyimide Copolymer with the Soft Segment ofMixed Polyethylene Adipates

The procedure was the same as that described in Example 1 except amixture of polyethylene adipate diol with a molecular weight of 500 (PEA500) and polyethylene adipate diol with a molecular weight of 2000 (PEA2000) at a molar ratio of 0.7 to 0.3 was used instead of only PEA 2000.The resulting membrane had a thickness of about 38 microns.

EXAMPLE 6 Pervaporation Results for the Polyimide Copolymer with theSoft Segment of Mixed Polyethylene Adipates

The resulting membrane described in Example 5 was evaluated in a similarway as described in Example 2. FIG. 8 shows the toluene/isooctaneselectivity and permeability for the polyimide copolymer with the softsegment containing a mixture of polyethylene adipate with a molecularweight of 500 (PEA 500) and polyethylene adipate with a molecular weightof 2000 (PEA 2000) at a molar ratio of 0.7 to 0.3. The soft segment hadan average molecular weight of about 950. This molecular weight was veryclose to that for the soft segment of the polyimide copolymer containingthe single polyethylene adipate, PEA 1000. In FIG. 8, the selectivityand permeability results for the polyimide copolymer with the PEA 1000soft segment and the polyurethane without Cl as shown in FIG. 7 arereproduced for comparison. As shown in FIG. 8, with about the samesoft-segment molecular weight of 1000, the polyimide copolymer with thesoft segment containing the mixed PEA 500 and PEA 2000 had lowerselectivity but higher permeability than that with the soft segment ofthe single PEA 1000. Presumably, this was due to the enhanced swelling(solubility) of the soft segment containing PEA 2000. In the similarbehavior as the polyimide copolymer with the PEA 1000 soft segmentdiscussed above, the polyimide copolymer with the soft segment of themixed polyethylene adipates had a nearly constant value for theselectivity and a slight increase for the permeability with increasingtemperature from 150° C. to 190° C. Also shown in FIG. 8, the polyimidecopolymer with the mixed polyadipates had higher selectivity but lowerpermeability than the polyurethane.

EXAMPLE 7 Synthesis of the New Copolymer, With PolyethyleneSuccinate/PMDA/MOCA at a Mole Ratio of 1/2/1, Containing 29 wt %Polyimide Hard Segment and 71 wt % Polyethylene Succinate Soft Segment

To 2.18 g (0.01 mole) of pulverized pyromellitic dianhydride (PMDA)heated (about 105° C.) under N₂ in a reactor was added 10 g (0.005) moleof polyethylene succinate diol with a molecular weight of about 2000(PES) with stirring. The temperature was about 105° C., and the stirringcontinued for about 5 hours to complete the end-capping step. To thereactor content was added 41.3 g of DMF, and the temperature was droppedto about 80° C. with stirring for about 0.5 hour. To this reactorcontent was added 1.34 g (0.005 mole) MOCA in 10 g DMF solutiondropwise. The solution was stirred at 80° C. for 2.5 hours. Then, aviscous solution resulted, which indicated the chain-extension reaction.The solution was then cooled to room temperature. The resulting solutioncontaining the copolymer with the polyamic acid hard segment and thepolyethylene succinate soft segment had suitable consistency forsolution casting in the preparation of membranes.

The resulting solution was centrifuged for about 5 minutes. Followingcentrifugation, a membrane was knife-cast onto a glass plate with aknife gap setting of 13 mils. DMF was allowed to evaporate from themembrane in a nitrogen box in a hood at ambient conditions over a periodof about 17 hours. The membrane was then dried in an oven at 90° C. for2 hours. The membrane was removed from the glass plate by soaking it ina water bath. Then, the membrane was dried at 120° C. for about 39hours. Finally, the membrane was heated to 300° C., maintained at thistemperature for 1.5 hours and then cooled to room temperature in thecuring step. The resulting membrane synthesized from PES/PMDA/MOCA at amolar ratio of 1/2/1 contained 29 wt% polyimide hard segment and 71 wt %polyethylene succinate soft segment, and it had a thickness of about 16microns.

EXAMPLE 8 Synthesis of the New Copolymer, With PolyethyleneSuccinate/PMDA/MOCA at a Molar Ratio of 1/3/2, Containing 40 wt %Polyimide Hard Segment and 60 wt % Polyethylene Succinate Soft Segment

To 1.05 g (0.0048 mole) of pulverized pyromellitic dianhydride (PMDA)heated (about 110° C.) under N₂ in a reactor was added 4 g (0.0024 mole)of polyethylene succinate diol with a molecular weight of 1667 (PES)with stirring. The temperature was about 110° C., and the stirringcontinued for about 4 hours to complete the end-capping step. To thereactor content was added 3 g of DMF, and the stirring was continued for1 hour at 110° C. An additional 37 g of DMF was added, and thetemperature was dropped to about 80° C. with stirring for about 0.5hour. To this reactor content was added 1.29 g (0.0048 mole) MOCA in 10g DMF solution at once. The stirring was continued for another hour.Then, 0.52 g (0.0024 mole) of PMDA and 5.9 g of DMF were added, thestirring was continued for another hour at 80° C. Then, a viscoussolution resulted, which indicated the chain-extension reaction. Thesolution was then cooled to room temperature. The resulting solutioncontaining the copolymer with the polyamic acid hard segment and thepolyethylene succinate soft segment had suitable consistency forsolution casting in the preparation of membranes.

The resulting solution was centrifuged for about 5 minutes. Followingcentrifugation, a membrane was knife-cast onto a glass plate with aknife gap setting of 12 mils. DMF was allowed to evaporate from themembrane in a nitrogen box in a hood at ambient conditions over a periodof about 17 hours. The membrane was then dried in an oven at 90° C. for2 hours. The membrane was removed from the glass plate by soaking it ina water bath. Then, the membrane was dried at 120° C. overnight.Finally, the membrane was heated to 300° C., maintained at thistemperature for 1.5 hours and then cooled to room temperature. Theresulting membrane synthesized from PES/PMDA/MOCA at a molar ratio of1/3/2 contained 40 wt % polyimide hard segment and 60 wt % polyethylenesuccinate soft segment, and it had a thickness of about 37 microns.

EXAMPLE 9 Pervaporation Results for Polyimide/Polysuccinate CopolymerMembranes

The resulting membranes described in Examples 7 and 8 were evaluated foraromatics/saturates separation with the feed mixture of 10 wt % toluene,40 wt % p-xylene, 20 wt % isooctane and 30 wt % n-octane in thepervaporation apparatus described above.

As shown in Table 2, for pervaporation at 150 ° C., thepolyimide/polysuccinate copolymer membrane containing 29 wt % of thepolyimide hard segment and 71 wt % of the soft segment of PES (which wassynthesized from PES, PMDA, and MOCA at a molar ratio of 1/2/1) had anoverall aromatics/saturates separation factor of 9 and a permeability(normalized flux) of about 600 Kg·μM/M² /day. This membrane had a muchhigher selectivity than the polyimide/polyadipate copolymer containingthe same hard segment and the soft segment of polyethylene adipate (PEA)with a molecular weight of 2000 (which was synthesized from PEA 2000,pyromellitic dianhydride and MOCA at a molar ratio of 1/2/1 as describedin Example 1). The former membrane had a lower permeability than thelatter. This polyimide/polysuccinate copolymer membrane had the sameselectivity but much higher permeability than the secondpolyimide/polyadipate copolymer membrane, shown in this table,containing the soft segment of PEA 500 (which was synthesized from PEA500, pyromellitic dianhydride and MOCA at a molar ratio of 1/3/2). Thepolyimide/polyadipate copolymer had a permeability of about 60 Kg·μM/M²/D.

                  TABLE 2                                                         ______________________________________                                        POLYIMIDE/POLYSUCCINATE GIVES                                                 IMPROVED SELECTIVITY/FLUX                                                                                  Flux                                                            Aromatics/Saturates                                                                         (Permeability)                                   Membrane       Seperation Factor                                                                           (Kg.μM/M.sup.2 /D)                            ______________________________________                                        Polyimide/Polyadipate                                                                        5             3000                                             (MW 2000)                                                                     Polyimide/Polyadipate                                                                        9              60                                              (MW 500)                                                                      Polyimide/Polysuccinate                                                                      9             600                                              (29/71)                                                                       Polyimide/Polysuccinate                                                                      10.7          200                                              (40/60)                                                                       ______________________________________                                    

The second polyimide/polysuccinate copolymer membrane, shown in thistable, containing 40 wt % of the polyimide hard segment and 60 wt % ofthe PES soft segment (which was synthesized from PES, pyromelliticdianhydride and MOCA at a molar ratio of 1/3/2) had a separation factorof 10.7 and a permeability of about 200 Kg·μM/M² /D. This membrane had ahigher selectivity but a lower permeability than the firstpolyimide/polysuccinate membrane with 29 wt % of the hard segment. Thus,increasing hard segment fraction enhances selectivity but reducespermeability owing to a lower degree of swelling for the soft segment inthe presence of penetrant molecules. This second polyimide/polysuccinatecopolymer membrane had higher selectivity and permeability than thepolyimide/polyadipate copolymer membrane with the soft segment of PEA500.

FIG. 9 shows the overall aromatics/saturates selectivity andpermeability results whereas FIG. 10 shows the toluene/n-octaneselectivity and permeability results for the polyimide/polysuccinatecopolymer membranes in the pervaporation of the model feed as a functionof temperature from 150° C. to 210° C. For the temperature rangeinvestigated, increasing hard segment fraction improves selectivity butdecreases permeability. As shown in these figures, these membranes wererun up to 210° C. and were stable at this temperature, which was themaximum temperature capability of the pervaporation apparatus. Thus,these membranes have exhibited the highest thermal stability among themembranes investigated. As shown in FIG. 11, these membranes show higherselectivity at a given permeability than polyimide/polyadipate andpolyurethane membranes.

EXAMPLE 10 Thin-Film-Composite Membrane of the Polyimide CopolymerContaining the Soft Segment of Polyethylene Adipate with a MolecularWeight of 2000

A thin-film-composite membrane of the polyimide copolymer containing thesoft segment of polyethylene adipate with a molecular weight of 2000(PEA 2000) was synthesized via the same procedure as described inExample 1 except (1) the solution was cast onto a microporous teflonsupport with about 50 micron thickness, 0.2 micron pores and 80%porosity instead of casting onto a glass plate and there was no need tosoak the thin-film-composite membrane into water as in the case ofExample 1 to release the membrane from the glass plate, and (2) themembrane was cured by heating from room temperature to 260° C.,maintaining it at this temperature for 5 minutes and then cooling it toroom temperature. The resulting membrane had a thickness of about 20microns excluding the microporous support, and it had apolyimide/polyadipate weight ratio of 25/75. The membrane was evaluatedfor aromatics/saturates separation with the feed mixture of 10 wt %toluene, 40 wt % p-xylene, 20 wt % isooctane and 30 wt % n-octane in thepervaporation apparatus described above. FIG. 12 shows toluene/isooctaneand toluene/n-octane separation factors and flux from 150° C. to 210° C.

EXAMPLE 11 Thin-Film-Composite Membrane of the Polyimide CopolymerContaining the Soft Segment of Polyethylene Adipate with a MolecularWeight of 3000

A thin-film-composite membrane of the polyimide copolymer containing thesoft segment of polyethylene adipate with a molecular weight of 3000(PEA 3000) was synthesized in the same way as described in Example 10except PEA 3000 was used instead of PEA 2000 and the membrane was curedat 260° C. for 12 minutes instead of 5 minutes. The resulting membranehad a thickness of about 8 micron excluding the microporous support, andit had a polyimide/polyadipate weight ratio of 18/82. The membrane wasevaluated for aromatics/saturates separation in the same way asdescribed in Example 10. FIG. 13 shows toluene/isooctane andtoluene/n-octane separation factors and flux from 150° C. to 210° C.

EXAMPLE 12 Effect of Membrane Thickness on Selectivity and Flux

A thin-film-composite membrane of the polyimide copolymer containing thesoft segment of polyethylene adipate with a molecular weight of 2000(PEA 2000) was synthesized in the same way as described in Example 10except (1) the end-capping step was carried out at about 140° C. forabout 1 hour and (2) the membrane had a thickness of about 7 micronsexcluding the microporous support. The resulting membrane had apolyimide/polyadipate weight ratio of 25/75.

We evaluated the membrane of 7 microns in the same way as described inExample 10 for the membrane of 20 microns in pervaporation of the feedof 10 wt % toluene, 40 wt % p-xylene, 20 wt % isooctane and 30 wt %n-octane. FIG. 14 shows toluene/isooctane and toluene/n-octaneseparation factors and permeability from 150° C. to 210° C. The thinnermembrane maintained the selectivity and permeability as the thickermembrane. The permeability result indicated that the flux for thepolyimide copolymer membranes is proportional to the reciprocal ofmembrane thickness as predicted from the Fick law. FIG. 15 shows thatthe 7 micron membrane had much higher flux (about 3 times) than the 20micron membrane.

EXAMPLE 13 Effect of Soft Segment Molecular Weight on Selectivity andFlux

Two polyimide copolymer membranes, (1) polyethylene adipate with amolecular weight of 500 (PEA 500) as the soft segment and (2)polyethylene adipate with a molecular weight of 1000 (PEA 1000) as thesoft segment, were synthesized via the same procedure as described inExample 8 except PEA 500 and PEA 1000 were used instead of polyethylenesuccinate and they were thin-film-composite membranes. That is, thesepolyimide copolymer membranes had the soft segment/PMDA/MOCA at a molarratio of 1/3/2. The first membrane had a thickness of about 35 micronsand the second had about 10 microns (excluding the support). Thesemembranes were evaluated for aromatics/saturates separation with thefeed mixture of 10 wt % toluene, 40 wt % p-xylene, 20 wt % isooctane and30 wt % n-octane in the pervaporation apparatus described above. FIG. 16shows that decreasing soft segment molecular weight increasesselectivity but decreases flux. As shown in this figure, thepolyimide/polyadipate copolymer membrane containing the soft segment ofPEA 500 had higher toluene/n-octane selectivity but lower flux than thatwith a soft segment of PEA 1000.

EXAMPLE 14 Selectivity-Permeability Relationship for PolyimideCopolymers with Various Polyadipate Molecular Weights

We synthesized a family of polyimide copolymer membranes with thevarious molecular weights of polyethylene adipate soft segments, PEA500, PEA 1000, PEA 2000, and PEA 3000 as described in Examples 10, 11and 13. The toluene/n-octane selectivity and permeability results shownin FIGS. 12, 13, and 16 are replotted in a log-log scale in FIG. 17. Asshown in this figure, selectivity correlates with permeability for thepolyimide copolymer membranes with various polyadipate molecularweights. The permeability values cover a wide range from 60 to 10000kg·μM/M² /D, i.e., over more than 2 orders of magnitude.

EXAMPLE 15 Selectivity-Permeability Relationship forPolyimide/Polyadipate Copolymers Cured at Different Conditions--Effectof Curing on Selectivity and Flux

A thin-film-composite membrane of the polyimide copolymer containing thesoft segment of polyethylene adipate with a molecular weight of 2000(PEA 2000) was synthesized in the same way as described in Example 10except it was cured at 300° C. for 1.5 hours instead of 260° C. for 5minutes. The resulting membrane had a thickness of about 28 micronsexcluding the microporous support, and it had a polyimide/polyadipateweight ratio of 25/75. The membrane was evaluated foraromatics/saturates separation in the same way as described in Example10.

FIG. 18 shows that increasing curing severity improves selectivity butreduces flux. In this figure, the toluene/isooctane selectivity and fluxresults for the polyimide/polyadipate copolymer membrane cured at 300°C. for 1.5 hours are compared with those for the membrane of the samecomposition but cured at 260° C. for 5 minutes, which are shown in FIG.12 (Example 10). The former membrane with more severe curing had higherselectivity but lower flux than the latter with less curing. FIG. 19shows the toluene/n-octane separation factors and flux results of thesetwo membranes. Again, the membrane with more severe curing had higherselectivity but lower flux than that with less curing.

We have replotted the selectivity and flux (permeability) results inFIG. 19 for these two membranes in FIG. 20 in the log-log plot describedabove. As shown in FIG. 20, selectivity correlates with permeability forthe polyimide/polyadipate copolymer membranes of the same composition,i.e., the same polyadipate molecular weight, but cured at differentconditions. The selectivity/permeability relationship agrees very wellwith that shown in FIG. 17 for the polyimide copolymer membranes withvarious polyadipate molecular weights, which is represented by the solidline in FIG. 20. As discussed above, FIG. 17 also includes the resultsfor the polyimide copolymer membrane of polyadipate MW 3000 cured at260° C. for 12 minutes, which is different from the curing conditionsused for the two membranes with polyadipate MW 2000. Therefore,selectivity correlates with permeability for polyimide/polyadipatecopolymer membranes with various polyadipate molecular weights anddifferent curing conditions.

EXAMPLE 16 Effect of Curing on Selectivity and Flux

A thin-film-composite membrane of the polyimide copolymer containing thesoft segment of polyethylene adipate with a molecular weight of 2000(PEA 2000) was synthesized in the same way as described in Example 12except the membrane was cured at 260° C. for 4 minutes instead of 5minutes. The resulting membrane had a thickness of about 7 micronsexcluding the microporous support, and it had a polyimide/polyadipateweight ratio of 25/75. This membrane was evaluated foraromatics/saturates separation in the same way as described in Example12. The toluene/isooctane selectivity and flux results for this membraneare shown in FIG. 21, and its toluene/n-octane selectivity and fluxresults are shown in FIG. 22. In these figures, the selectivity and fluxresults for the polyimide/polyadipate copolymer membrane cured at 260°C. for 4 minutes are compared with those for the membrane of the samecomposition and thickness but cured at 260° C. for 5 minutes, which areshown in FIG. 14 (Example 12). THe latter membrane with slightly moresevere curing had slightly higher selectivity but lower flux than theformer with slightly less curing. These results have again reinforcedour finding that increasing curing severity improves selectivity butreduces flux.

EXAMPLE 17 Thermal Stability of Copolymer Membranes

We characterized thermal decomposition temperatures for polyimidecopolymer and chlorinated polyurethane membranes via thermogravimetricanalysis. As shown in FIG. 23, the polyimide/polyadipate copolymermembrane described in Example 1 had a thermal decomposition temperatureof about 410° C. in air, and the polyurethane membrane with the hardsegment containing 6 chlorines shown in FIG. 3 had a thermaldecomposition temperature of about 315° C. Although both polyimidecopolymer and chlorinated polyurethane membranes contained the same softsegment of polyethylene adipate with a molecular weight of 2000 (PEA2000), the polyimide copolymer membrane had a much higher thermaldecomposition temperature than the chlorinated polyurethane membrane.

We also characterized thermal decomposition temperatures forpolyethylene adipate diols with the molecular weights of 2000 and 4000(PEA 2000 and PEA 4000) in air and nitrogen. FIGS. 24 and 25 show thethermal decomposition temperatures for PEA 2000 and PEA 4000 in air,respectively, and FIGS. 26 and 27 give the thermal decompositiontemperatures for PEA 2000 and PEA 4000 in nitrogen, respectively. Asshown in these figures, the effects of PEA molecular weight andenvironment (air or nitrogen) on the thermal decomposition temperatureof PEA diol were insignificant. The thermal decomposition temperaturefor the PEA diols was about 300° C.

From the results discussed above, the thermal decomposition temperaturefor the polyimide/polyadipate copolymer membrane was much higher (about100° C.) than those for the chlorinated polyurethane membrane (with thesame polyadipate soft segment) and the polyadipate diol. This suggestedthat the decomposition temperature was affected by the hard segment. Thedecomposition temperature for the polyadipate diol may be influenced byits terminal hydroxyl groups. The urethane linkage between the hard andsoft segments in the polyurethane membrane is less thermally stable thanthe ester linkage between the hard and soft segments and the imidelinkages of the hard segment in the polyimide copolymer membrane.

Polyimide copolymer membranes had a temperature stability of about 210°C. in pervaporation with the feed mixture described above. FIG. 28 showsthat the polyimide copolymer membrane with the soft segment of PEA 3000described in Example 11 (FIG. 13) had stable toluene/isooctane andtoluene/n-octane separation factors and flux for at least 6 days at 210°C. in pervaporation with the feed mixture. The membrane was evaluated at150°, 170°, 190° and 210° C. for one day at each temperature. Then, themembrane was kept in the pervaporation apparatus at 210° C. for 6 days.As shown in this figure, the selectivity and flux after 6 days at 210°C. were about the same as those obtained initially at 210° C. Thetemperature stability of the polyimide copolymer membranes (about 210°C.) was much higher than that of about 170° C. for the chlorinatedpolyurethane membrane (6 Cl), which had the highest stability among thepolyurethane membranes investigated.

What is claimed is:
 1. A method comprising: separating aromatics fromfeeds which are mixtures of aromatics and non-aromatics by selectivelypermeating the aromatic hydrocarbon through a thin membrane including acopolymer composition comprising the hard segment of a polyimide and thesoft segment of an oligomeric aliphatic polyester, wherein saidpolyimide is derived from a dianhydride having between 8 and 20 carbonand a diamine having between 2 and 30 carbons and said oligomericaliphatic polyester is a polyadipate, a polysuccinate, a polymalonate, apolyoxalate or a polyglutarate.
 2. The method of claim 1 wherein saiddianhydride is an aromatic compound.
 3. The method of claim 1 whereinsaid diamine is selected from the group consisting of phenylene diamine,methylene dianiline (MDA), methylene di-o-chloroaniline (MOCA),methylene bis(dichloroaniline) (tetrachloro MDA), methylenedicyclohexylamine (H₁₂ -MDA), methylene dichlorocyclohexylamine (H₁₂-MOCA), methylene bis(dichlorocyclohexylamine) (tetrachloro H₁₂ -MDA),4,4'-(hexafluoroisopropylidene)-bisaniline (6F diamine),3,3'-diaminophenyl sulfone (3,3'DAPSON), 4,4'-diaminophenyl sulfone(4,4'DAPSON), 4,4'-dimethyl-3,3'-diaminophenyl sulfone(4,4'-dimethyl-3,3'DAPSON), 2,4-diamino cumene, methylbis(di-o-toluidine), oxydianiline (ODA), bisaniline A, bisaniline M,bisaniline P, thiodianiline, 2,2-bis propane (BAPP), bis sulfone (BAPS),4,4'-bis(4-aminophenoxy) biphenyl (BAPB), 1,4'-bis(4-aminophenoxy)benzene (TPE-Q), and 1,3-bis(4-aminophenoxy) benzene (TPE-R).
 4. Themethod of claim 2 wherein said aromatic compound is selected from thegroup consisting of pyromellitic dianhydride, 3,3',4,4'-benzophenonetetracarboxylic dianhydride,4,4'-(hexafluoroisopropylidene)-bis(phthalic anhydride),4,4'-oxydiphthalic anhydride, diphenylsulfone-3,3',4,4'-tetracarboxylicdianhydride, and 3,3'4,4'-biphenyl-tetracarboxylic dianhydride.
 5. Themethod of claim 1 wherein said polyester is polyethylene adipate.
 6. Themethod of claim 1 wherein said polyester is polyethylene succinate. 7.The composition of claim 1 wherein said diamine is methylenedi-o-chloroaniline.